Survey and comparison of focal plane MTF measurement techniques

A sensor's Modulation Transfer Function (MTF) characteristics determine the upper limits of the image quality, i.e. the image resolution or sharpness. The MTF describes the image quality in terms of contrast as a function of spatial frequency, normalized to unity at zero spatial frequency. Unfortunately, characterization of the MTF of semiconductor-based focal plane arrays (FPAs) has typically been one of the more difficult and error-prone performance testing procedures. Discussed in this paper are several commonly used techniques for measuring the MTF in the visible and IR wavelength regions. A brief description of the physical nature of FPAs is presented. This description will show that, because the MTF varies as a function of illumination wavelength and can vary as a function of illumination intensity, the conditions and techniques used for MTF measurement should be chosen based on the ultimate application of the FPA. Trade-offs between complexities in measurement and complexities in analysis as applied to an MTF characterization will be discussed as well as the importance of proper sensor calibration and the measurement of the optical test system's MTF. A discussion of analysis validation, including the effects of noise, using simulated image data is given. Following this, a sample comparison of a sensor's MTF curves as measured by a variety of techniques is presented. Opto-mechanical issues that can significantly influence the quality of an MTF measurement are also discussed.